1. Field of the Invention
The present invention relates generally to CMOS image sensors and particularly to a method and apparatus for increasing retention time in image sensors having an electronic shutter.
2. Description of the Related Art
Active Pixel Sensors (APS) sensors are now becoming viable due to their diminishing pixel size. Image sensors that are manufactured using a CMOS process exhibit many advantages over those image sensors manufactured using a CCD based process. These advantages include affordability, space savings, a reduction in power consumption, a windowed readout, increased image quality, increased speed and performance, and the availability of on-chip processing.
A CMOS image sensor can typically include an electronic shutter that mimics a mechanical shutter. A mechanical shutter is employed in cameras to control the exposure of light to film. For example, both the shutter speed and the aperture size of the shutter influence the amount of light that is permitted to reach the film. Since there is typically no mechanical shutter to control the exposure of the sensor to light in a CMOS imager, an electronic shutter is employed to in effect control the exposure to light by limiting the amount of time a storage element is allowed to charge.
An electronic shutter typically includes a pass transistor and a storage element (e.g. capacitor). The capacitor is employed to store a voltage value that represents the amount of light incident to the light sensor. The pass transistor is controlled by a sample signal that in effect controls the charge transfer time of the capacitor. The charge transfer time is simply the time that the capacitor is charged by the photodetector. The value stored by the capacitor of the electronic shutter is read out to be manipulated or stored. The readout rate of a pixel is fairly constant for typical architectures and processes (i.e., the readout rate cannot be improved beyond a specified limit).
Prior art CMOS image sensors have the following disadvantages. First, the image sensor array is limited to a small pixel format. For example, a typical CMOS image sensor array includes 512 by 512 pixels. As processing and manufacturing techniques improve, and as smaller submicron geometries are achieved, pixel sizes decrease and hence, larger arrays are viable.
As the size of image arrays increases, the time before a pixel value is read out increases accordingly. Consequently, the size of the storage element for each pixel needs to be increased to maintain an accurate pixel value for read out.
Furthermore, prior art CMOS image sensors typically operate under a row mode. A row mode is a mode in which the image sensor array is exposed to light one row at a time, and each row is read out one row at a time.
There are many advantages to designing an image sensor having a frame mode instead of or in addition to a row mode. A frame mode is a mode in which all the pixels in the image sensor array are exposed during the same time, and then each row is read out one row at a time. The advantages of frame mode are 1) increased accuracy of the scene content; and 2) uniformity in exposure time and scene content.
In row mode, the electronic shutter retention time need only be a time from when the electronic shutter receives the value from the first photosensor in a row to the time that the value in the last photosensor of that row is read out. The retention time in a frame mode is much greater since each pixel must retain the pixel value from the time the value is transferred to the electronic shutter of that pixel to the time that pixel is read out. Obviously, pixels that are in the later rows must retain their pixel value longer than those pixels in earlier rows since the pixels in the earlier rows are read out sooner than the later pixels. The pixel in the last row of the array suffers the worst case retention time since these pixels must wait for pixels in all the other rows to be read out before their values are read out. When the pixels fail to accurately retain their values, undesirable artifacts, such as image smear, appear in the image.
Accordingly, there remains an unmet need in the industry for a method and apparatus for increasing the electronic shutter retention time in CMOS image sensors so that the CMOS image sensors can support frame mode operation and large pixel formats.
In a pixel having an electronic shutter, a method of increasing the retention time of the electronic shutter is disclosed. A reset signal is employed to drive a diode node to a predetermined voltage immediately after integration is completed. A sample signal is employed to control a pass gate. The sample signal includes a state where the sample signal is a negative voltage.